Devices for ground-resistance detection

Abstract

A ground-resistance detection device, which is coupled to a power system including a first power line, a second power line, and a protective earth, includes a ground-resistance detection circuit and a controller. The ground-resistance detection circuit includes a first input node and a second input node. The first input node is coupled to either the first power line or the second power line. The second input node is coupled to a ground terminal. The ground-resistance detection circuit generates a DC output voltage according to the voltage of the first input node, the voltage of the second input node, and the DC reference voltage. The controller determines the ground resistance between the protective earth and the ground terminal according to the DC reference voltage and the DC output voltage.

What is claimed is: 
     
       1. A ground-resistance detection device, coupled to a power system, wherein the power system comprises a first power line, a second power line, and a protective earth, comprising:
 a ground-resistance detection circuit, comprising a first input node and a second input node, wherein the first input node is coupled to either the first power line or the second power line, and the second input node is coupled to a ground terminal, wherein the ground-resistance detection circuit generates a DC output voltage (V OUT ) according to a voltage of the first input node, a voltage of the second input node, and a DC reference voltage (V REF ), wherein the ground-resistance detection circuit further comprises: 
 a first operation amplifier, comprising a first positive input node and a first negative input node, wherein the first operation amplifier is supplied by a DC supply voltage, and compares a voltage of the first positive input node with a voltage of the first negative input node to generate the DC output voltage at an output node; and 
 a controller, determining a ground resistance (R PE ) between the protective earth and the ground terminal according to the DC reference voltage and the DC output voltage. 
 
     
     
       2. The ground-resistance detection device of  claim 1 , wherein the power system is a single-phase three-wire system, and the power system comprises a live wire, a neutral wire, and the protective earth, wherein the first input node is coupled to either the live wire or the neutral wire. 
     
     
       3. The ground-resistance detection device of  claim 1 , wherein the power system is a two-phase three-wire system, and the power system comprises a first live wire, a second live wire, and the protective earth, wherein the first live wire comprise a first phase and the second live wire comprises a second phase, wherein the first input node is coupled to either the first live wire or the second live wire. 
     
     
       4. The ground-resistance detection device of  claim 1 , wherein the power system is a three-phase four-wire system, and the power system comprises a first live wire, a second live wire, a third live wire, and the protective earth, wherein the first live wire comprise a first phase, the second live wire comprises a second phase, and the third live wire comprises a third phase, wherein the first input node is coupled to the first live wire, the second live wire, or the third live wire. 
     
     
       5. The ground-resistance detection device of  claim 1 , wherein the ground-resistance detection circuit further comprises:
 a first resistor, comprising a first resistance (R 1 ) and coupled between the first input node and the first negative input node; 
 a second resistor, comprising a second resistance (R 2 ) and coupled between the first negative input node and the output node; 
 a third resistor, comprising a third resistance (R 3 ) and coupled between the second input node and the first positive input node; 
 a fourth resistor, comprising a fourth resistance (R 4 ) and coupled between a reference input node and the first positive input node, wherein the DC reference voltage (V REF ) is supplied to the reference input node; 
 a first capacitor, coupled between the output node and the first negative input node; and 
 a second capacitor, coupled between the first positive input node and the reference input node, wherein the first input node comprises an AC signal, and the first capacitor and the second capacitor are configured to select a bandwidth to filter out the AC signal. 
 
     
     
       6. The ground-resistance detection device of  claim 5 , wherein the ground-resistance detection circuit further comprises:
 a first clamp circuit, coupled to the first negative input node and configured to provide a clamp protection for the first negative input node. 
 
     
     
       7. The ground-resistance detection device of  claim 5 , wherein the controller obtains the ground resistance from an equation:
     R   PE =( R   3 ·( R   1   +R   2 )· V   REF   −R   1 ·( R   3   +R   4 )· V   OUT )/(( R   3   +R   4 )· V   OUT   −R   3   ·V   REF ).
 
 
     
     
       8. The ground-resistance detection device of  claim 1 , further comprising:
 an AC-voltage detection circuit, comprising a third input node and a fourth input node, wherein the third input node is coupled to the first power line, and the fourth input node is coupled to the second power line, wherein the AC-voltage detection circuit generates a detection voltage according to an AC voltage between the third input node and the fourth input node, wherein the detection voltage comprises a DC-part voltage (V DC ) and an AC-part voltage, wherein the controller determines the AC voltage (V M ) according to amplitude of the AC-part voltage, and determines the ground-resistance according to the DC-part voltage and the DC output voltage. 
 
     
     
       9. The ground-resistance detection device of  claim 8 , wherein the AC-voltage detection circuit comprises:
 a second operation amplifier, comprising a second positive input node and a second negative input node, wherein the second operation amplifier receives a DC supply voltage and compares a voltage of the second positive input node with a voltage of the second negative input node to generate the detection voltage at a detection voltage node; 
 a fifth resistor, comprising a fifth resistance (R 5 ) and coupled between the third input node and the second negative input node; 
 a sixth resistor, comprising a sixth resistance (R 6 ) and coupled between the second negative input node and the detection voltage node; 
 a seventh resistor, comprising a seventh resistance (R 7 ) and coupled between the fourth input node and the second positive input node; 
 an eighth resistor, comprising an eighth resistance (R 8 ) and coupled between a reference input node and the second positive input node, wherein the DC reference voltage is supplied to the reference input node; 
 a third capacitor, coupled between the detection voltage node and the second negative input node; and 
 a fourth capacitor, coupled between the second positive input node and the reference input node, wherein the third capacitor and the fourth capacitor are configured to filter out high-frequency noise. 
 
     
     
       10. The ground-resistance detection device of  claim 9 , wherein the AC-voltage detection circuit comprises:
 a second clamp circuit, coupled to the second negative input node and configured to provide a clamp protection for the second negative input node; and 
 a third clamp circuit, coupled to the second positive input node and configured to provide the clamp protection for the second positive input node.